1. Field of the Invention
The present invention relates to integrated circuit lead frames and methods of production thereof. In particular, this invention relates to plastic lead frames with a conductive coating or material contained therein used for packaging integrated circuits and methods of manufacturing the same.
2. State of the Art
Integrated circuit (IC) chips are enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection to printed circuit boards. During a manufacturing process, the IC chip is typically attached to a die paddle of a conventional lead frame or suspended from the lead fingers of a leads-over-chip (LOC) lead frame using an adhesive such as epoxy or double-sided tape, and subsequently encapsulated with a dense and rigid plastic by a transfer molding process. In essence, the lead frame forms the backbone of the molded plastic IC package.
Lead frames typically perform many functions such as: (1) a holding fixture that indexes with tool-transfer mechanisms as the package proceeds through various assembly operations, (2) a dam that prevents plastic from rushing out between leads during the molding operation, (3) a chip attach substrate, (4) a support matrix for the plastic, and (5) an electrical and thermal conductor from chip to board.
Traditionally, lead frames are fabricated from a strip of sheet metal by stamping or chemical milling operations. There are many different metal alloy compositions which are commercially available for producing lead frames. For example, Rao R. Tummala and Eugene J. Rymaszewski, "Microelectronics Packaging Handbook," Table 8-4, 1989, provides 16 different alloys available from 9 different manufacturers. Lead frame material selection depends on many factors such as cost, ease of fabrication, strength, thermal conductivity, and matched coefficient of thermal expansion (CTE). A close match of CTE between the silicon die and the lead frame is required to avoid chip fracture from differential expansion rates.
The most widely used metal for lead frame fabrication is Alloy 42 (42% Nickel-58% Iron). Alloy 42 has a CTE near silicon and good tensile strength properties. The disadvantage of Alloy 42 is that it has low thermal conductivity. Since the lead frame is the main conduit by which heat flows from the chip to the environment and printed circuit board, this can have a profound effect on the package thermal resistance after prolonged device operation.
A layered composite strip, such as copper-clad stainless steel, was developed to emulate the mechanical properties of Alloy 42 while increasing thermal conductivity. However, copper-clad stainless steel is somewhat more expensive to manufacture than Alloy 42. When manufacturing copper-clad stainless steel lead frames, the cladding is accomplished by high-pressure rolling of copper foil onto a stainless steel strip, followed by annealing the composite to form a solid-solution weld. While copper alloys provide good thermal conductivity and have a CTE near that of low-stress molding compounds, there is a substantial CTE mismatch with respect to silicon.
While numerous alloys have been developed to solve problems with thermal conductivity, CTE mismatch, and strength, other important factors such as ease of fabrication and cost have not improved as readily.
Conventional methods for making lead frames for integrated circuit devices are described in U.S. Pat. No. 3,440,027. The use of a plastic support structure in a method of forming metal lead frames is described in U.S. Pat. No. 4,089,733 (hereinafter the "'733" patent). The plastic support structure of the '733 patent solves the problem of deformed and misaligned lead fingers resulting from stress during the bonding process by supporting the lead fingers with a plastic structure. However, the '733 patent requires a metal lead frame in addition to the plastic support structure with its attendant costs. A method of manufacturing multi-layer metal lead frames is disclosed in U.S. Pat. No. 5,231,756 (hereinafter the "'756" patent). The '756 patent provides an improvement in aligning power and ground planes for use in a multi-layer lead frame where such planes are necessary. However, the number of steps required to manufacture such multi-layer lead frames will not solve the problem of decreasing costs. In short, none of the related art appears to disclose methods of producing low-cost lead frames made from materials not structurally based on metal.
Since packaged ICs are produced in high volumes, a small decrease in the cost per packaged IC can result in substantial savings overall. Accordingly, there is a need in the industry for a low cost plastic lead frame with suitable characteristics for IC packaging.